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The stability of cosmological event and Cauchy horizons of spacetimes associated with plane symmetric domain walls is studied. It is found that both horizons are not stable against perturbations of null fluids and massless scalar fields; they are turned into curvature singularities. These singularities are lightlike and strong in the sense that both the tidal forces and distortions acting on test particles become unbounded when these singularities are approached.
Wang et al. (Tue,) studied this question.